This invention relates to speed control switches for use with portable electric motor-driven tools and more particularly to improvements in such speed control switches.
A large number of patents have been granted for various speed control switches and for the various improvements in parts of such switches. U.S. Pat. No. 3,467,801 issued to Matthews on Sept. 16, 1969 and No. 3,691,322 also issued to Matthews on Sept. 12, 1972, both disclose reversing switches intended for use with motor speed controls of the type with which the present invention is concerned. U.S. Pat. No. 3,415,966 issued to Matthews on Dec. 10, 1968, discloses an improved bridge contactor for use with such motor speed controls. U.S. Pat. No. 3,761,788 issued to Matthews et al, on Sept. 25, 1973, discloses an electronic switch module generally of the thick film hybrid circuit type in which resistance elements are printed onto a surface of ceramic housing forming half of the switch module. U.S. Pat. No. 3,886,340 issued to Bittel on May 27, 1975, discloses an arrangement for positioning a switch module in a housing to help align the variable speed circuitry with the trigger actuator of the speed control unit.
Each of the above listed patents represents an attempt to solve various problems which have been encountered in providing low cost, reliable, variable speed controls for electric motor-driven tools. For example, the patents dealing with reversing switches both illustrate the need for such switches and the complexity of devices which have been previously used in attempts to make such devices small enough to fit within the housing of the motor-driven tools. It can be seen that there is still a need for smaller less complicated and more reliable reversing switches for this application.
The above-referenced U.S. Pat. No. 3,415,966 illustrates a reliability problem in the mechanical switch part of these devices. The contactor arrangement taught by that patent includes a central caming projection which lifts up one end of the bridging contactor in breaking the electrical contact and prevents the smearing of plastic material onto the stationary contact when contact is made. In addition, a groove is provided in the contact face of the bridge contactor which is said to aid in removal or elimination of contamination from the stationary contact surface. Such contamination can prevent the making of a good contact and thereby cause the switch to over-heat or prevent the motor from operating. In addition, the groove in the contact face is said to aid in the extinction of arcs which occur when the switch contact is broken. This arcing is known to have a generally destructive effect on all parts of the switch. Due primarily to the arcing damage, most commercial devices when tested on automated life test equipment driving an actual motor load fail in the range of ten to fifteen thousand operating cycles. Occasionally such devices may provide up to 50,000 cycles of operation prior to failure. Thus, if can be seen that further improvement to the switch contact arrangement, which would reduce contact damage from either build-up of contaminants or arcing, would be desirable and would lengthen the lifetime of the device.
This arcing problem has been found to be a particular problem in the common half-wave control variable speed circuit devices such as that disclosed in the above-referenced U.S. Pat. No. 3,761,788. The half-wave speed control devices employ a silicon controlled rectifier instead of a bi-directional device and thereby can provide variable voltage outputs only in the nominal range of zero volts up to one half of the power line voltage. When the trigger actuator of such a device is fully depressed, the mechanical switch contactor bypasses the electronic circuitry and connects full voltage across the motor thereby increasing the supply voltage from one half to full value. When the trigger actuator is released, the voltage therefore drops from a full line voltage to one half value at the time a mechanical switch contact is broken. It is well known that the inductance of a motor essentially always causes an arc to occur upon the breaking of that switch contact. Reduction of arcing caused in this manner would improve the lifetime of this type of relatively inexpensive half-wave motor speed control circuitry.
It has also been found difficult, in actual practice, to provide reproducable trigger actuated motor speed control devices in the sense of providing particular output voltages and resulting motor speeds for given trigger actuator positions. The above-referenced U.S. Pat. No. 3,886,340 illustrates this problem and an attempt to solve it. The variable speed circuitry of the speed control switch provides an output voltage which is proportional to the value of a variable resistance. The resistor value is controlled by the position of a sliding contactor which is linked to the trigger actuator. It is desirable that the variable resistance be accurately positioned relative to the trigger actuator so that a given actuator position will always provide a particular output voltage and resulting motor speed. This positioning has been found to be difficult and as a result various devices provide different turn-on speeds. This turn-on speed is related to the output voltage of the device at the time that the mechanical switch contacts make the connection of the load motor to the variable speed circuitry. The solution taught by the Bittel Patent is to provide projections in the unit housing which mate with portions of the circuitry module to hold the two in a fixed relative position. It can be seen that other sources of misalignment, such as the exact position of the printed resistors on the substrate, will still cause variations in turn-on speed, etc. . . . from unit to unit.